It’s easy to understand why San Francisco 49ers linebacker Chris Borland walked away from an NFL career and a $3 million contract with his faculties intact if you consider the latest numbers from the nation’s largest brain bank.

Borland did not want to wind up as a prime donor candidate for the bank, where recent research has found brain disease in 87 of 91 former NFL players. In total, 96 percent of deceased NFL players and 79 percent of deceased football players tested positive for chronic traumatic encephalopathy, or CTE.

CTE is a progressive, degenerative disease found in some athletes and others with a history of repetitive brain trauma that can cause dementia, depression, memory loss, lack of impulse control and a decrease in motor skills. CTE can only be diagnosed after death.

CTE was present in the brains of Junior Seau, Dave Duerson and Ray Easterling, who committed suicide. Baltimore tight end John Mackey had it, and Steelers center Mike Webster. Former Dolphin Mark Duper, who had at least four concussions during his career and now struggles through periods of anger and depression, suspects he has CTE.

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The Veterans Affairs-Boston University-Concussion Legacy Foundation Brain Bank has diagnosed CTE in 131 of 165 men who played football before they died — either in high school, college, as a semipro or in the NFL. Forty percent of the pros were linemen, suggesting cumulative subconcussive trauma poses the most risk. The ongoing study is being conducted in conjunction with PBS’ Frontline and its Concussion Watch project.

The numbers told Borland that the link between hits to the head and brain disease can no longer be denied, that what’s happening to retired NFL players can no longer be dismissed as senility or Alzheimer’s disease. A battered brain does not necessarily recover. Too often, a football player loses his mind.

Former Dolphin Mark Duper, who had at least four concussions during his career and now struggles through periods of anger and depression, suspects he has CTE.

“What the evidence keeps telling us is that somebody we played football with is probably going to develop CTE,” said Chris Nowinski, who played for Harvard and became a professional wrestler. “The earlier you start, the longer you play and the more hits you take, the more damage you suffer.”

The evidence does not show that 96 percent of NFL players will die with CTE, Nowinski emphasized. Much about the disease remains unknown, and most of the players who donated their brains to science suspected something was gravely wrong, which skews the sample. But the percentages are striking, especially since they are not decreasing even as the number of brains examined has increased sixfold.

“Three million kids are playing football before they get to high school, and that’s where we can focus on prevention,” said Nowinski, whose Concussion Legacy Foundation hammers home additional numbers, such as the fact that 3.8 million concussions occur per year and only one in six are diagnosed. “We need to stop hitting children in the head for sport. A lifetime of brain trauma opens the door to terrible things.”

The NFL does not want to hear the latest numbers, nor is the league looking forward to release of the film Concussion in December. Will Smith plays Dr. Bennet Omalu, the forensic pathologist who found CTE in the brain of Webster, who was homeless and depressed before he died. Omalu’s research was ripped apart by NFL doctors who served on the league’s since-disgraced brain injury committee, which concluded that “professional football players do not sustain frequent repetitive blows to the brain on a regular basis.”

The NFL was hoping its concussion crisis would go away with its $1 billion settlement of a lawsuit by thousands of former players who alleged the league hid or downplayed the harm caused by head trauma. The league reports that concussions were down 25 percent last season because of “rule changes, advanced sideline technology and expanded medical resources.”

But NFL retirees who are appealing the settlement as inadequate and full of loopholes that exclude many players will use the new data to advocate for more compensation.

“This study is another brick in the wall linking CTE and head trauma in pro football players, which was left as an open question in the settlement,” said Michael V. Kaplen, a critic of the settlement who has filed a friend of the court brief on behalf of the Brain Injury Association of America. “The NFL says it is setting aside $1 billion but that doesn’t mean they’ll pay it. There are so many catch-22s that players in many instances will receive very little or nothing. We’re arguing that the science of brain injury is being corrupted.”

Borland decided to get out of the game before it was too late. He’d read about Webster, Duerson, Easterling, heard about the problems of Tony Dorsett, Jim McMahon, Brett Favre. He consulted with CTE experts, tallied the numbers and decided he didn’t want to be one.